The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply; it includes aromatic compounds believed to cause cancer; and contains sulfur and nitrogen-containing compounds that can adversely affect the environment, for example by producing acid rain.
Alternative sources for developing combustible liquid fuel are desirable. An abundant source is natural gas. Natural gas is often plentiful in regions that are uneconomical to develop because of a lack of local markets for the gas or the high cost of transporting the gas to remote markets. In such areas, the natural gas, which comprises mostly methane, is flared off rather than used, thus wasting the natural gas and adding undesirable combustion products to the atmosphere. An alternative is to convert the natural gas to a liquid fuel or other chemical product for local usage and more cost-effective transportation to remote markets.
The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less health and environmental impact than conventional petroleum liquids based fuels.
Refining processes create materials which can foul process equipment, such as reactors and heaters. For example, Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible liquid fuel, among other products. A feature associated with Fischer-Tropsch chemistry is that it produces a number of reactive species such as olefins and alcohols that, under certain conditions such as high temperatures, tend to form materials that will foul (adhere to) equipment surfaces, damaging or diminishing the effectiveness of heat exchangers, furnaces, catalytic reactors and the like. It is believed that some of the formed materials are polymers or other large molecules. Polymerization of olefins is known to occur where crude-oil derived hydrocarbons are exposed to oxygen-containing environments such as in storage tanks, and are then heated in a typical hydroprocessing plant to near reaction temperatures. For example, naphtha derived from refinery coking operations tends to form these products. While not wishing to be bound by theory. Fischer-Tropsch products are believed to behave similarly when heated because of the presence of olefins and oxygen-containing compounds, even without exposure to further oxygen sources.
Equipment fouling is known to be mitigated by stripping the hydrocarbon feed stream with hydrogen or fuel gas, primarily to remove oxygen. However, when the oxygen is in the form of alcohols rather than as oxygen gas, as is the case with Fischer-Tropsch products, the oxygen cannot be removed by feed stripping.
Sacrificial guard beds have been employed upstream of main catalytic reactors to collect polymerized products before passage through and, thus, fouling of heat exchangers, furnaces and other equipment. However, the use of such guard beds increases manufacturing costs significantly. In addition, such guard beds may not always be effective in prevention of heat exchange surface fouling.
It is also known to pre-heat hydrocarbon feeds to hydroprocessing reactors in shell and tube heat exchangers using the reactor effluent. Both the hydrocarbon feed (usually in the form of a pumpable liquid) and a large quantity of hydrogen-rich gas (e.g. greater than 750 Standard Cubic Feet per Barrel (SCFB)) are combined before entering the heat exchanger. In this case, the hydrogen-rich gas acts as a velocity-maintaining agent (usually on the tube-side of the exchanger) to avoid the deposition of particulate matter in the feed, thus preventing fouling of the heat exchangers and other equipment in the feed pre-heat train.
It would be advantageous to provide additional means of protecting pre-heat equipment that handles, for example, Fischer-Tropsch synthesis and refinery coking products, which minimizes the formation of fouling products. The present invention provides such methods.